1. Field of the invention
The invention relates to a method for transmitting, via a transmission medium With a plurality of virtual, asynchronously time-divided transmission channels, a flow of data supplied to that transmission medium and consisting of data cells which are each transmitted via one of those transmission channels. Each of the data cells comprises a group of control words with one or more control words as well as a group of information words with one or more information words, the group of control words comprising a control word with a channel indication, indicating via which transmission channel the relevant data cell has to be transmitted, and the state of a counter being kept up to date for each transmission channel, which state has a certain fixed minimum value, and which is on the one hand decreased proportional to the time, and which is on the other hand increased proportional to the number of data cells with a channel indication, relating to said transmission channel. The state of the counter is compared with a threshold value when a data cell with a channel indication designating said transmission channel arrives at the beginning of the transmission medium at a certain moment of arrival, after which the data cell will be let through to the transmission medium located downstream, when the state of the counter is less than said threshold value, or it will not be let through to the transmission medium located downstream, when the state of the counter is more than or equal to said threshold value.
2. Problems
The above-mentioned, rather new method of data transmission--generally named ATM (=Asynchronous Transfer Mode)--which is particularly useful for wide band ISDN, is applied in quick packet switched networks, in which the information is contained in packets, called (data) cells, each consisting of one or more information words, and comprising not only those information words, but also a "header" with (inter alia) a control word indicating the transmission channel via which the cell has to be transmitted to its destination.
The transmission means and the switching means can have a capacity in the range of some Mbit/s up to many Gbit/s. The ATM-technique enables the network administrator to distribute the available capacity in an extremely flexible way.
A user could, however, try to supply a larger flow of cells to the network than the capacity allocated to him permits. Because of the flexibility of the ATM-technique this does not yet need to lead immediately to problems at the entrance of the network. An overload can, however, occur somewhere else on the route in the network due to which the traffic of other users could also be disturbed. In order to avoid such a situation it will be necessary to monitor the entrance, thus preventing the passage of cells of a certain user which exceed the capacity allocated to him. In reference (1) the monitoring of the entrance of ATM-networks is discussed by considering two concepts, which are both based on a method which is known under the name of "leaky bucket" method or "leaky integrator" method. According to this method the state of a counter is kept up to date (with a certain minimum value), which state is on the one hand decreased by a first constant value at set times and on the other hand increased by a second constant value whenever a data cell arrives. Subsequently the state of the counter is compared with a threshold value, after which the relevant data cell will be transmitted only if the state of the counter is less than that threshold value. Both the long-term average of the number of cells per unit of time as well as the number of cells which may be transmitted (during a short time) above that average (for example the number of cells which may be supplied immediately following one another) can be monitored by a mechanism of the aforesaid type. Only those data cells which do not exceed the maxima set will be transmitted.
In the aforesaid reference it is suggested to provide a "leaky bucket" device as described above for each network user (in other words, for each virtual transmission channel). The advantage of this is that each transmission channel will be monitored. A great practical problem is, however, that since the number of virtual transmission channels can be very large, e.g. 2.sup.16 (=65,536), the number of monitoring devices has to be equally large, which is very unattractive from a technical and commercial point of view.
In said reference it is also suggested to provide, for the whole group of network users (in other words, for the whole group of virtual transmission channels), a monitoring device as mentioned above for monitoring the joint users as a group. The advantage of this is that a much smaller number of monitoring devices has to be provided, but the disadvantage is that the various transmission channels cannot be monitored individually, which causes inter alia the problem that when the maximally allowed cell frequency is exceeded, it cannot be determined which of the various users is responsible for this, and consequently of which user the transmission of data cells has to be refused.